1. Field of the Invention
This present invention relates to the use of special tin salts of organic acids as additives to prevent skinning in air drying binders.
2. Description of the Prior Art
Oils and binders which undergo three-dimensional crosslinking by oxidation in the presence of oxygen (preferably from the air) at room temperature or at higher temperatures by the addition of driers (for example, metal soaps of transition metals; the solutions of driers in organic solvents or also water are known as "drier preparations") and, thus, are converted from the liquid phase into the solid phase may form a skin on their surface during storage in open or closed containers. This crosslinking which proceeds before the actual application of the lacquer is highly undesirable since it complicates handling of the lacquer because the skin must first be removed.
In extreme cases skinning may proceed to such an extent that none of the lacquer in the container may be used. Skinning also considerably reduces the storage stability of the lacquer or coating composition. This amounts to a marked degradation in the quality of the coating composition.
One example of skinning is caused by an accumulation of certain driers at the surface. Incorporation of the driers in the skin has a negative effect on the drying characteristics of the remainder of the lacquer.
Accordingly, it is necessary and also well known to add substances to the lacquer which inhibit the reaction with atmospheric oxygen at the (lacquer) liquid surface, i.e., in the container. These substances are known as antioxidants, antiskinning agents or additives. A list of known compounds may be found, for example, in H. Kittel, Lehrbuch der Lacke und Beschichtungen, Colomb Verlag, 1976 or also W. Kurze in Ullmann, Lexikon der Chemie, volume 8, pp. 19 et seq. (5.sup.th edition).
The most important classes of prior art compounds for the lacquer and printing inks industry are phenolic compounds and oximes. Phenolic antiskinning agents often considerably retard surface drying, such that they may be used alone only in special formulations. To the contrary oximes, such as methyl ethyl ketoxime, delay surface drying only slightly by virtue of their volatility. A disadvantage of this class of compounds resides in the toxicological properties thereof, which means that elaborate precautions are required during the application and formulation of these lacquers and coating compositions.
One particular application of methyl ethyl ketoxime involves spraying it onto high viscosity coating compositions in cans or similar containers. In this case the coating composition without an antioxidant is packaged in a container, sprayed with the antioxidant and then sealed under reduced pressure. This method is in particular used for printing inks. The advantage of this process is the small quantity of antioxidants required (based on the entire quantity of coating composition) and the extremely long possible storage life of such lacquers. However, it is necessary to spray the methyl ethyl ketoxime, which requires increased safety measures.
An object of the present invention is to provide antioxidants (antiskinning agents) which exhibit no negative effects on the drying behavior and properties, of the applied film of the corresponding coating compositions and in particular have no disadvantageous toxicological properties. It is an additional object of the present invention to provide antioxidants which may be applied only onto the surface of a coating composition which has already been formulated and packaged in a container for storage.
These object may be achieved with the antioxidants according to the invention which is described hereinafter.